In the case of anisotropic plasma etching of silicon according to the method described in German Patent No. 42 41 045, a certain amount of harmful polymer deposits and/or sulfur deposits occur in the vacuum components of the etching system, i.e., particularly in the turbopump, exhaust lines, and backing pump used. This results from the decomposition of the employed process gas sulfur hexafluoride (SF6) excited by plasma with the release of fluorine resulting in elementary sulfur or from the formation of polymerizable monomers from the process gas C4F8 or C3F6, which is preferably used for side wall passivation, forming Teflon-like films.
To avoid deposits in the exhaust area, it was previously proposed in German Published Patent Application No. 198 26 382 that, oxygen be added in the proportion of 3 vol. % to 40 vol. % to the etching gas sulfur hexafluoride only during the etching steps according to the method in German Patent No. 42 41 045, i.e., not during the deposition steps or polymerization steps. This prevents the deposition of sulfur, for example, in the exhaust area and, in particular, in the area of the turbopumps located there, while also ensuring that the anisotropic plasma etching process is not significantly impaired.
However, the method according to German Published Patent Application No. 198 26 382 has a disadvantage in that, in the process according to German Patent No. 42 41 045, it is not possible to suppress any deposits other than sulfur present in the exhaust gas, for example polymers, since the oxygen admixture can only take place during the etching steps, but not during the polymerization steps during which polymer-forming compounds in the exhaust area are used. While it is therefore possible to suppress sulfur deposits in the exhaust area according to the teaching of German Published Patent Application No. 198 26 382, it is not possible to avoid unwanted polymer deposits.
In a departure from the general rule requiring the avoidance of oxygen in the polymerization steps, however, it has been found in certain special applications that the addition of small amounts of oxygen is advantageous even during this phase. In high-rate anisotropic etching processes using a ceramic ICP (inductively coupled plasma) process chamber, the addition of 3 to 10 sccm O2, preferably 5 sccm O2, to 200 sccm C4F8 greatly suppresses the formation of carbon micromasks. This addition is unnecessary when using quartz glass chambers, since in this case the wall made of SiO2, on its own accord, releases a small but sufficient amount of oxygen in this magnitude, as a result of reactions with fluorine radicals resulting in SiF4. Note, however, that the addition of 10 sccm O2 to, for example, 200 sccm C4F8 is the maximum amount tolerated during the polymerization steps without producing side effects that impair the quality of the generated etching profile. In any case, this amount is insufficient for suppressing deposits in the exhaust area of the vacuum system.
According to another known method of suppressing deposits in the exhaust area of a vacuum system, a turbopump is provided with a “temperature management system”, and a cold trap (cryotrap) is positioned downstream from the turbopump. Specifically, this means that the turbopump is heated to a relatively high operating temperature between 70° C. and 100° C. during vacuum system operation, with the result that no harmful polymer deposits or sulfur deposits initially occur therein. To achieve this, however, the turbopump must have magnetic bearings, since conventional ball bearings are not approved for these temperatures and are very quickly destroyed by bearing wear. This also means relying on a special and very expensive turbopump design.
In addition, turbopump heating prevents the deposition of the polymerizable species only within the pump, which merely transfers the problem to the exhaust line downstream from the turbopump, in particular to the backing pump located there, unless an additional cold trap is provided directly downstream from the turbopump to bind the harmful species in which the polymerizing agents are deposited and thus removed from the exhaust gas. However, this cold trap constitutes an additional cost factor in conjunction with the already expensive, special turbopump. In addition, the cold trap must be emptied regularly and its contents disposed of, which is not easily done in light of the toxic compounds that are frequently deposited.